High velocity spraying processes based on combustion of oxygen-fuel mixtures (HVOF) or air-fuel mixtures (HVAF) allow coatings to be sprayed from variety of materials. HVOF and HVAF processes may generally produce sonic and supersonic gas jets including combustion products of the oxygen-fuel or air-fuel mixtures. High quality coatings can be sprayed at a high level of efficiency when the temperature of the combustion products is high enough to soften or melt the particles being sprayed and the velocity of the stream of combustion products is high enough to provide the required density and other coating properties. Different materials require different optimum temperatures of the sprayed particles in order to provide an efficient formation of high quality coatings. Higher melting point materials, such as cobalt and/or nickel based alloys, carbides and composite materials, may often require relatively high temperatures in order to soften the particles to a level sufficient to efficiently form high quality coatings.
Some of the parameters affecting the available range of temperatures and velocities available from the combustion products are combustion pressure, types of fuel and oxidizer and ratio of fuel/oxidizer flow rates. Commonly used fuels may include gaseous and liquid hydrocarbon fuels like propane, propylene, MAPP gas, kerosene. Hydrogen may also be used as a fuel. Liquid fuels may provide some advantages over gaseous fuels. The use of liquid fuels may be less expensive than gaseous fuels and may be more easily fed into combustion apparatus at high pressure by using pumps or pressurized tanks. Some of gaseous fuels, for example, propane, are supplied in tanks at relatively low pressure. A tank of a gaseous fuel at low pressure may require pre-heating in order to provide a spraying gun with high pressure gaseous fuel. The pre-heating isn't attractive from safety standpoint.
Combustion devices and other parts of combustion apparatus may require cooling because of high temperatures of combustion. Cooling, however, may result in heat losses from the combustion apparatus to the cooling media. This heat loss may be a factor that can affect the efficiency of the process, for example by influencing the temperature and velocity of a combustion jet. Heat losses may depend, at least in part, on the intensity of the cooling and the surface areas of the combustion apparatus that are being cooled by a cooling media.
According to some designs, compressed air or oxygen is fed through air passages surrounding the combustion chamber and the barrel/nozzle assembly in order to cool these parts. The compressed air is then fed from the passages into the combustion chamber and is used as an air supply for the combustion process. This “regenerative” heat exchange may be economical and may reduce heat losses from the combustion. Oxygen has a relatively low cooling efficiency, however, and cooling using only oxygen may not be sufficient to prevent an HVOF system, which may generally operate at a higher temperature than an HVAF system, from overheating.
Oxygen/fuel mixtures may achieve high combustion temperatures, in some cases reaching temperatures of 3000 degrees C. or higher. To protect the apparatus from damage due to these extreme temperatures, water is commonly used as a cooling media for oxygen/fuel mixtures. In addition to the use of water cooling systems, combustion chambers for burning oxygen/fuel mixtures, as well as other components that will be exposed to high temperatures, are often manufactured from copper or copper alloys. Very efficient cooling may be achieved using water as a cooling medium in combination with copper or copper alloy components. Unfortunately, such efficient cooling may result in relatively large heat losses, especially in combustion systems having large internal surface areas and/or numerous turns in the path of combustion products.